Abstract Among older adults, walking speed is an excellent predictor of health, happiness, and longevity. Unfortunately as adults mature, their preferred walking speeds rapidly decline as their corresponding metabolic costs increase; changes that impair older adult?s ability to navigate their communities. Based on recent scientific findings, I posit that the age-related decline in tendon stiffness contributes to the increased metabolic cost and reduced walking speeds of older versus young adults. That is because compared to youthful (stiffer) tendons, aged (compliant) tendons shift the in-series muscle contractile dynamics to presumably less economical states, which I theorize relates to slower and less economical locomotor movement in older versus young adults. Accordingly, this project?s 1st Aim seeks to uncover the relationships between tendon stiffness, in-series muscle dynamics (i.e. operating lengths and shortening velocities), and the corresponding metabolic cost of cyclic contractions. To fulfill this aim, I will systematically alter older adult soleus muscle and Achilles tendon dynamics during cyclic, non-invasive isolated muscle contractions. I will characterize the corresponding physiological responses using a battery of physiological measurements, highlighted by the use of 1) ultrasound imaging that enables me to track muscle and tendon movement ?underneath the skin? and 2) open-circuit respirometry that enables me to quantify participant metabolic rates. I hypothesize that effectively reducing Achilles tendon stiffness elicits less economical in-series muscle dynamics. The results of this aim will uncover fundamental relationships between aged-musculoskeletal systems and the corresponding metabolic costs of performing locomotor tasks. For the 2nd Aim, I will determine how soleus muscle contractile dynamics independently affect older adult metabolic rates during walking. To do this, I will employ passive (custom footwear) and active (ankle exoskeleton emulator) assistive devices that each systematically alter older adult soleus dynamics during walking, while I assess relevant biomechanical and physiological measures: muscle/tendon dynamics, metabolic cost, muscle activation, stride kinetics and kinematics. I hypothesize that the assistive device that optimizes the interplay of soleus operating length and shortening velocity will minimize the metabolic cost of walking and improve older adult preferred walking speed. The results of Aim 2 will reveal the optimal soleus muscle dynamics to improve older adult walking economy and speed. Study implications may be used to inform assistive device design, surgical procedures, and rehabilitation/exercise programs aimed to enhance older adult leg function and walking performance. Altogether, this study is set to comprehensively test the space of cyclic muscle contractions to reveal the fundamental links between tendon stiffness, muscle dynamics, and energetics, with special regard to older adult mobility.